The field of art to which this invention pertains is the process of crosslinking polygalactomannans.
Use of thickening agents and gelling agents for the purpose of increasing viscosity of water or water solutions has been known for many years. Among most widely used thickeners are those compounds generally characterized as hydroxylated polymers which contain vicinal bis-hydroxyl groups. The most readily available of these polymers are the polygalactomannans and their derivatives. These polymer gums form viscous solutions in water which function is accomplished by absorption of large amounts of water by the polymer or by dissolution of the polymer gum in water or both. The degree of thickening and the viscosity of the water to which the polymer is added will depend upon the quantity and kind of polymer used. A wide range of solution properties can be produced by the selection of particular gums and by the adjustment of the quantities used and the temperature of the reaction medium. These polymer gums have found wide application in the textile, food, pharmaceutical, explosive, mining and oil field industries.
A useful property of the polygalactomannan gums is their ability to form firm gels when crosslinked by various agents. Well-known crosslinking agents are borates, pyroantimonates and chelates of the transition metals, e.g., titanium. Heretofore, the use of titanium compounds has been limited to highly acidic media or to pH's of 6 or higher.
In U.S. Pat. No. 4,033,415, it is stated that titanium tetrachloride will not cause crosslinking of a gelling agent at a pH of more than 2; however, when added to a medium containing an aqueous liquid and a gelling agent, the titanium tetrachloride reacts to form the intermediate hydroxide in hydrochloric acid in sufficient quantities to adjust the pH of the medium to less than 2 where crosslinking will occur. Another disadvantage of using titanium tetrachloride is that it reacts violently when added to aqueous media.
In U.S. Pat. No. 3,301,723, it is stated that bases must be used with transition metal compounds in order to obtain practical crosslinking reactions. Without base, compounds of only a few of the transition metals, namely, vanadium, titanium and zirconium compounds, will cause crosslinking to occur. However, compounds of these metals alone do not produce extensive gelation unless they are used in high concentrations, i.e., usually at least in an order of magnitude greater than the concentrations that are effective for crosslinking when base is present. In addition to being less economically attractive, such concentrations of crosslinking agents can cause degradation of the gel structure or other undesirable side reactions. Furthermore, unless base is present, wide variations in the extent and rate of crosslinking may occur so that reproducible control of the process becomes very difficult.
In U.S. Pat. No. 4,324,668, the statement is made that crosslinking agents containing metals, such as aluminum, chromium, titanium, boron, lead, zinc, tin and antimony, do not function satisfactorily in aqueous highly acidic media.
The present invention provides a process for crosslinking polygalactomannans with titanium compounds in aqueous systems wherein the crosslinked system has a pH of about 2.5 to about 6.